The present invention relates to a double-focusing mass spectrometer having a Wien filter and an instrument using such a mass spectrometer as its second mass-selective device for conducting mass spectrometry (MS)/mass spectrometer (MS).
Mass spectrometry involving detecting daughter ions dissociated from parent ions in a field-free region is quite useful in elucidating the molecular structures of complex organic compounds. Heretofore, such spectrometry has been carried out by either linked scan method where the electric field and the magnetic field of a double-focusing mass spectrometer are varied in an interrelated manner, or a MS/MS method using tandem arrangement of two mass spectrometers.
In a linked scan method, daughter ions dissociated from metastable ions in the field-free region between an ion source and an electric field are detected. When metastable ions dissociated into daughter ions, it is thought that the daughter ions travel at the same velocity as the parent ions. The kinetic energy of a particle having a mass of m and a velocity of v is given by mv.sup.2 /2 and, therefore, produced daughter ions possess kinetic energies proportional to their masses. For this reason, the energy of the daughter ions lie in a wide range.
When such a group of daughter ions is analyzed with a conventional double-focusing mass spectrometer having a cylindrical electric field and a magnetic sector, daughter ions which can pass through the cylindrical field are only ions having energies lying within about +5% of a given value, or within an energy range of about 10%. Consequently, the linked scan method, in which the two fields are varied in an interrelated manner, is adopted for enabling mass analysis of daughter ions having a wide range of energies, i.e., a broad range of masses.
In conducting MS/MS, the first mass spectrometer sorts out only parent ions having a given mass. The parent ions dissociate into daughter ions in the field-free region between the first and second mass spectrometers. The resulting daughter ions are introduced into the second mass spectrometer and a mass spectrum of the daughter ions is obtained.
Where a conventional double-focusing mass spectrometer which consists of a cylindrical electric field and a magnetic sector is employed as the second mass selective device to obtain daughter-ion spectra with high resolution, since daughter ions possess a wide range of energies, a linked scan method must be adopted.
In this way, when daughter ions are analyzed by a double-focusing mass spectrometer having a cylindrical electric field and a magnetic field, it has been heretofore imperative that both fields be altered in an interrelated way. Where both fields are scanned in this fashion to provide a spectrum, all ions not impinging on the ion detector are eliminated. This leads to a deterioration in the sensitivity. Hence, there is a limit to enhancement of the sensitivity.